El Niño is back in the news, much as it was last year at this time. But this year, El Niño conditions in the tropical Pacific have intensified, and some climate scientists think the outcome this winter could be the return of much-needed rain. Hopeful media reports are describing the growing El Niño as a potential drought buster.

How solid are these predictions and should we count on a wet winter? Unfortunately, El Niño is an unreliable predictor of winter storminess. Although some of our wettest years have occurred during El Niño events, some very dry years have also occurred.

El Niño is the name given to a climatic pattern that originates in the Pacific tropics, and involves both the ocean and the atmosphere. It is defined by unusually warm upper-ocean waters in the central and eastern tropical Pacific, and is linked to slackened trade winds. El Niño conditions usually persist for several months and recur, irregularly, roughly every two to seven years. El Niño (and its opposite phase, a cool tropical Pacific condition known as La Niña) is Earth’s strongest and most important short-term climate variation because of its global reach: it disturbs climate and ecosystems in the tropics but also unleashes altered atmospheric patterns well beyond the tropics.

One of the regions it affects, usually during the winter, is California. Some El Niño events are strong enough to impact the North Pacific jet stream, which steers winter storms into California. Because the bulk of our seasonal precipitation occurs in a handful of strong winter storms the additional El Niño events make a big difference to the state’s water supply.

But El Niño often produces strong regional differences in precipitation. In many El Niño years, Southern California can be unusually wet, but the state's important water supply areas in Northern California are often not—sometimes they are even unusually dry. When this happens, the water supply benefits of El Niño are limited. What’s more, in some El Niño years the entire state remains dry. The figure below shows how widely precipitation can range, with or without El Niño.

So why are climate scientists so energized about El Niño this summer?

El Niño conditions, once established, tend to last for several months. Beginning this spring and continuing through this summer, scientists have observed unusual heat build-up in the upper layers of the tropical Pacific. Climate models are pointing to a moderate to strong El Niño through the summer and into the fall. But summer and early fall are the dry season in California, and El Niño conditions at this time will not make much of a difference in the ongoing drought. These same models indicate that El Niño conditions are likely to persist into the coming winter, which is key to shifting the jet stream and increasing the number of winter storms. Historically, unusually strong El Niño events have been linked to record wet years in California. Two of the wettest water years on record in the state—1983 and 1998—occurred during very powerful El Niño conditions. So while the warmth this year is impressive, so far the 2015 El Niño is not in the same league as the extraordinary cases of 1983 and 1998, and the uncertainties over its intensity into next winter remain considerable.

As tempting as it is to hope this El Niño will take us off the hook for planning for a fifth year of drought, it would be unwise to bet on this, given the uncertainties. With reservoirs and groundwater at historic low levels after four consecutive dry, warm years, a single wet year is unlikely to erase the drought. Rather, it is prudent to plan now for continued impacts of our long dry spell. Major relief would be a pleasant surprise, but for now, continuing our efforts to conserve will pay off even if the hard rains come.

Chart note: This graph shows annual winter precipitation in the Sierra Nevada in relation to the Southern Oscillation Index, a measure of El Niño. Typically, the more negative the value, the stronger the El Niño, with greater global impacts on weather.